There are various types of leisure vehicles equipped with a multi-cylinder engine. Some types of leisure vehicles are, for example, a motorcycle, an all terrain vehicle, and a personal watercraft (PWC). These vehicles are configured so that output power from the engine is transmitted to a propulsion mechanism such as a wheel or a water jet pump to generate a propulsion force of the vehicle. Typically, the engine is equipped with a throttle valve control device. The throttle valve control device is configured to control the output power from the engine by opening/closing a throttle valve of the engine to adjust an intake amount of air supplied to a combustion chamber of the engine.
Japanese Published Unexamined Patent Application SHO62-162759 discloses a multi-cylinder engine equipped with a multi-throttle valve, for a purpose of reducing resistance of intake air to the engine. The multi-throttle valve includes a plurality of throttle valves each of which is provided in respective air-intake passage extending to a combustion chamber of each cylinder. Typically, the multi-throttle valve is of an interlink system or has a structure such that a plurality of throttle valves are rotatably supported by a common valve shaft and, thus, all of the throttle valves are integrally opened and closed to the same degree.
Further, there are several types of driving systems for the throttle valve control. One type of the driving systems is, for example, a system such as disclosed in Japanese Examined Utility Model Application SHO63-39406, in which an operator directly performs an open/close operation of a throttle valve through a cable (also referred to as a cable-driven type). Another type of the driving systems is an electronic control system in which the throttle valve is configured to be driven by an electric motor (also referred to as a motor-driven type). Still another type of the driving system is a combined system of the cable-driven and motor-driven types (also referred to as a double-throttle type).
The double-throttle type has a configuration in which the cable-driven type throttle valve (main valve) and the motor-driven type throttle valve (sub-valve) are arranged on a downstream side and on an upstream side, respectively, in a single air-intake passage. In this configuration, an air-intake amount is coarsely adjusted by the main valve according to a throttle operation by the operator, and the coarsely adjusted air is further fine-adjusted by the electronically-controlled sub-valve so that an optimized amount of the intake air is supplied into the combustion chamber of the engine.
In some cases with a four-cycle engine, as the throttle is gradually opened from a state in which the throttle valve is in a fully-closed state and a negative engine torque is generated, a rate of increase in the engine torque greatly changes with respect to a rate of increase in the throttle opening at a certain amount of the throttle opening. This is undesirable in terms of improving the riding comfort of the operator during the acceleration of the vehicle from an engine idle state.
Further in a multi-cylinder four-cycle engine, such great change in the engine torque may also occur in each cylinder, even if the multi-throttle valve of any of the above conventional driving systems is used. The overall engine torque is a sum of the outputted torques produced by all of the cylinders. Therefore, the above-mentioned change in the engine torque becomes noticeable for the engines with many cylinders, such as four cylinders or six cylinders, and may cause a difficulty in an improvement in the operator's riding comfort.
Typically, such engine torque characteristics are difficult to correct by merely adjusting an injecting amount of fuel to be mixed with the intake air, an injecting timing of the fuel, an ignition timing, and the like.
Meanwhile, in order to obtain a good operating feel of a throttle operating portion, such as a twist grip throttle control (TGTC), during acceleration/deceleration of the vehicle, it is desirable to improve a response of the throttle valve control device, that is, a response of the valve shaft. For this purpose, a configuration in which the electric motor is arranged so that the output shaft of the motor is coaxially aligned with the valve shaft, and the output shaft is directly coupled with one end portion of the valve shaft, is advantageous. However in this case, a dimension of the throttle valve control device in the valve shaft direction becomes large.
Further in order to improve the response of the throttle valve control device, a response of the motor against an input signal to the motor may be improved. This improvement may be achieved by reducing a radial dimension of the motor so as to reduce a moment of inertia of a rotor of the motor around the output shaft. However, in order to ensure a certain amount of torque from the motor, if the radial dimension is reduced, an axial dimension has to be increased instead. Accordingly, in the configuration in which the valve shaft and the output shaft are directly coupled to each other as described above, the dimension of the throttle valve control device in the valve shaft direction may further increase.
This consequently makes it difficult to mount the engine equipped with such a throttle valve control device especially on a vehicle with a relatively narrow engine mounting space, such as a motorcycle, an all terrain vehicle (ATV), and a personal watercraft (PWC), and this is undesirable.
For example, if the throttle valve control device is applied to a motorcycle equipped with an inline multi-cylinder engine, the throttle valve control device may be arranged so that the valve shaft is directed toward the width direction of the motorcycle. Accordingly, if the size of the throttle valve control device is increased, the size of the motorcycle needs to be increased in the width direction thereof. This size increase is undesirable if the riding comfort of the operator who rides on the motorcycle gripping a body of the motorcycle with his/her knees is taken into consideration.
Instead, the electric motor may be coupled with the valve shaft through a gear train including a bevel gear and the like to avoid the increase in the size of the throttle valve control device in the width direction of the vehicle. However, this configuration maynot aid in the improvement of the response of the throttle valve control device because the gear train is relatively large in mass and, thus, has a large moment of inertia.
Meanwhile, when the operator (rider) makes a turn (especially, a tight turn) on a motorcycle, the operator closes the throttle to decelerate the motorcycle before entering the corner and, then, opens the throttle to accelerate the motorcycle as approaching the end of the corner. In this case, when entering the corner, the operator must keep the throttle closed for a while and, then, the operator must open the throttle so as to match the throttle-opening operation accurately with a speed of the motorcycle.
If the motorcycle is equipped with the motor-driven throttle valve, the electric motor is configured to actuate the throttle valve directly based on the detection value from a throttle positioning sensor. Therefore, a timing of the throttle-opening operation and an amount thereof by the operator that matches the speed of the motorcycle must be accurately performed at all times.